1. Field of the Invention
The present invention relates to infrared imaging system calibration methods and shutter mechanisms used therein and, more particularly, to a calibration method that relies on an absolute measurement of scene radiance (thereby requiring a baseline measurement of zero radiance), and a shutter mechanism operable under cryogenic conditions to measure zero radiance by providing the imaging system with a temporary view of some object possessing zero radiance.
2. Description of the Background
Infrared imaging systems are highly effective, surveillance and/or diagnostic tools. For example, their ability to “see” radiation wavelengths outside of the visible spectrum may be utilized to sense the movements of humans and animals in the dark (i.e. during nighttime hours), or to call attention to the imminent failure of a mechanical or electrical device by pinpointing the location of an inappropriate heat source. Infrared imagers are also sometimes used as radiometers, devices that measure the radiant output of a source. In such uses the imager must be calibrated in terms of absolute radiance units since radiometric imagers are often to understand the behavior of other infrared sensor types (infrared missile seekers, etc.), and these measure the total absolute scene radiance (that radiance arising from all sources external to the sensor system).
The measurement of absolute radiance is somewhat difficult and usually involves the measurement of a known reference source relative to the system under test. This is a relative measurement, but since it is tied to a reference source of known radiance, the absolute quantity may be evaluated. Specifically, absolute radiance can be measured by taking the difference between the scene radiance signal and the reference radiance signal, and assuming that the camera zeroth signal (the signal resulting from just the camera) is unchanging. This can work for a wide variety of measurements where the scene has either positive or negative contrast with the reference. However, the approach can lead to difficulty, since it is possible that for negative contrast cases the scene radiance signal can fall to the camera zero signal level without the operator being aware of it (it is, after all, an unknown quantity). It is also possible that for certain parametric configurations (gain states) the camera will not be able to encompass both radiance values (reference and target). In other words the camera runs out of dynamic range.
In light of the above, it would be better to have a calibration method that relied on camera characteristics alone to make the measurement, e.g., relying on an absolute measurement of scene radiance (thereby requiring a baseline measurement of zero radiance). This approach has not heretofore been pursued because to effectively implement it the imaging system must be provided with a temporary view of some object possessing zero radiance. To create the temporary view, an object that possesses zero radiance must temporarily fill the field-of-view of the focal plane array of the imager. This requires a shutter mechanism that moves in front of the imaging system aperture, then returns to a non-occluding position, under cryogenic conditions.
Camera shutter mechanisms are well-known, but to effectively maintain an object with “zero” radiance the temperature must be very low and the shutter must be a “cold shutter.” Specifically, to effectively remove all radiance from an object in the midwave (3–5 μm) and long wave (8–12 μm) bands that typical imaging systems operate in, that object's temperature must be below 90K (−183° C.). While temperatures of that magnitude may be achieved by applying liquified, or nearly-liquified, gas to the object, or by placing the object in interplanetary space (which also exposes the object to a nearly perfect vacuum), conventional camera shutter mechanisms do not function in such environments.
The present inventor is not the first to address the need for shutter assemblies, or apparatus designed to open and close the aperture of an optical instrument, that are operable when subjected to cryogenic conditions (e.g. very low component temperatures). Other examples of shutter mechanisms possessing these attributes may be found in U.S. Pat. No. 5,384,661 to Geyer et al., U.S. Pat. No. 5,258,874 to Bajat et al., and U.S. Pat. Nos. 5,128,796 and 4,995,700 to Barney et al.
U.S. Pat. No. 5,384,661 to Geyer et al. discloses an articulated device for space vehicles for temporarily sealing the aperture of a space-based optical instrument. The device includes a fixed support element for mounting on the vehicle and an auxiliary element such as a temporary sealing flap mounted to pivot with respect to the fixed support element around a hinge axis between a first configuration (e.g. wherein the aperture is sealed) and a second configuration. The auxiliary element is attached, by a frangible controlled-break component for making temporary connections, to a hinge shaft whose rotation between the first and second configuration is controlled by an actuator. A torsional resilient return device is secured to the auxiliary element and to the shaft while resiliently biasing the auxiliary element to rotate it around the shaft in a predetermined direction. The shaft is substantially loosely engaged in journal bearings that are rigidly fastened to the auxiliary element.
U.S. Pat. No. 5,258,874 to Bajat et al. discloses a device for movement control of a mobile element that moves with respect to a support between two extreme positions. The invention includes an arm that pivots around a rotation axis which supports the mobile element; an elastic torsion rod aligned along the axis, secured respectively to the arm and to the support at axially offset points; two mechanical stops mounted on the support which determine two extreme angular configurations of the arm in which the element is in its extreme positions, the elastic torsion rod tending to bring the arm into a predetermined one of these configurations; two pair of complementary magnetic latching elements mounted either on the support or on the arm, the complementary units of either pair being adapted to be opposite when the arm is in one or the other of the extreme configurations; and launching members mounted on the support designed to give a launching impetus to the arm from each to the other of the extreme configurations.
U.S. Pat. Nos. 5,128,796 and 4,995,700 to Barney et al. disclose magnetically operated shutter mechanisms that will function in cryogenic or cryogenic zero gravity environments to selectively block radiation such as light from passing through a window to a target object such as a mirror or detector located inside a cryogenic container such as a Dewar. The mechanism includes a shutter paddle blade that is moved by an electro-magnetically actuated torquing device between an open position where the target object is exposed to ambient radiation or light, and a closed position where the shutter paddle blade shields the ambient radiation or light from the target object. The purpose of the shuttering device is to prevent the mirror or other target object from being directly exposed to radiation passing through the window located on the side wall of the Dewar, thereby decreasing or eliminating any temperature gradient that would occur within the target object due to exposure to the radiation. A special nylon bearing system is utilized to prevent the device from binding during operation and the paddle blade is also thermally connected to a reservoir containing the cryogen to further reduce the internal temperature.
Unfortunately, each of the above-described, prior art devices is relatively large and complex, and is not adaptable for use in small or medium format cryogenically cooled infrared cameras. Moreover, the prior art mechanisms appear to be prone to temperature variations which would affect their ability to remain fully functional in extreme environments.
There remains a significant need for a cryogenic shutter mechanism that, when actuated, provides the infrared radiation sensor (imaging system) with a view of a very low temperature surface in order to calibrate the zero radiation reference level in the imaging system. To the best of the knowledge of the present inventors, no such apparatus exists, and no one has endeavored to implement a calibration method that relies on an absolute measurement of scene radiance (thereby requiring a baseline measurement of zero radiance using a cryogenic shutter mechanism as described to provide the imaging system with a temporary view of some object possessing zero radiance).